Tomographic imaging methods are characterized in that internal structures of an examination object can be investigated without the need for invasive interventions to be performed on the object. A possible type of tomographic image generation resides in the acquisition of a number of projections of the object to be examined from different angles. A two-dimensional sectional image (slice) or a three-dimensional volume image of the examination object can be computed from the projections.
Computed tomography is one example of such a tomographic imaging method. Methods for scanning an examination object by way of a CT system are generally known. For example, typical methods employed in such cases are circular scans, sequential orbital scans with patient feedthrough, or helical scans. Other types of scan that are not based on circular movements are also possible, such as e.g. scans using linear segments. Absorption data of the examination object are acquired from different recording angles with the aid of at least one x-ray source and at least one oppositely located detector, and the thus collected absorption data or projections are computed by way of appropriate reconstruction methods into sectional images or slices through the examination object.
In order to reconstruct computed tomographic images from x-ray CT datasets of a computed tomography device (CT scanner), i.e. from the acquired projections, a method referred to as filtered back-projection (FBP) is currently employed as the standard procedure. Following the data acquisition a so-called “rebinning” step is normally performed in which the data generated by way of the beam spreading out from the source in the shape of a fan are reordered in such a way that the data are present in a form as if the detector had been impinged upon by x-ray beams converging in parallel onto the detector. The data are then transformed into the frequency domain. Filtering takes place in the frequency domain and subsequently the filtered data are back-transformed. A back-projection onto the individual voxels within the volume of interest is then performed with the aid of the thus re-sorted and filtered data. On account of the approximate mode of operation of conventional FBP methods, however, there are problems with so-called low-frequency cone beam artifacts and helical artifacts. Furthermore, in conventional FBP methods the image sharpness is coupled to the image noise. The higher the sharpness attained, the higher also is the image noise, and vice versa.
The FBP method belongs to the group of approximate reconstruction methods. There also exists the group of exact reconstruction methods, though these are hardly used at the present time. The iterative methods, finally, form a third group of reconstruction methods.
A problem occurring more and more as the number of detector rows increases, i.e. with increasing detector width, is scattered radiation. It is namely possible that an x-ray quantum, instead of being absorbed by the examination object, is scattered, i.e. deflected in terms of its direction. This means that a specific detector element also measures x-ray quanta which do not originate from the beam which connects the x-ray source to the respective detector element. This effect is referred to as forward scattering. It leads to undesirable artifacts in the reconstructed CT images.
There also exist CT devices having two x-ray sources, called dual-source devices. If both x-ray tube assemblies are operated with the same x-ray spectrum, this increases the temporal resolution of the CT images considerably. This is because the data acquisition time is halved owing to the two x-ray sources. This is desirable in particular in the case of moving examination objects. On the other hand it is also possible to operate the two x-ray sources at different acceleration voltages and consequently with different x-ray spectra, such that a dual-energy scan is performed. This enables meaningful conclusions to be drawn concerning the composition of the recorded tissue.
The presence of scattered radiation is a well-known problem in the case of dual-source scans also. In addition to the above-described forward scattering, cross scattering also occurs with dual-source devices. This means that radiation of an x-ray source which is scattered at the surface or on the inside of the examination object arrives at the detector which is not assigned to the x-ray source. This is undesirable because only the evaluation of the transmitted radiation of the x-ray source assigned to the respective detector is relevant.